Ontogeny and distribution of Fc gamma receptors in the human placenta. Transport or immune surveillance?

The human fetus acquires maternal IgG via the chorioallantoic placenta. Utilising antibodies against 3 characterised subtypes of IgG receptor (Fc gamma R) expressed by human leucocytes, we show by confocal immunofluorescence microscopy that these molecules are also expressed by cells of the placenta. Fc gamma RI (CD64) is expressed by undifferentiated mesenchymal or fibroblast cells of 1st trimester and term chorionic villi. Punctate immunoreactivity for Fc gamma RII (CDw32) is found on capillary endothelial cells of term and 1st trimester villi. Fc gamma RIII (CD16) expression is observed in the trophoblast surrounding chorionic villi that forms the functional 'barrier' between mother and fetus. In 1st trimester villi this receptor is associated with a population of marginated vesicular inclusions of the syncytiotrophoblast. In term villi the receptor is concentrated in the apex of the syncytiotrophoblast, suggesting a possible role in the maternofetal transmission of passive immunity. All 3 subtypes of receptor are expressed by Hofbauer cells. We have been unable to demonstrate these receptors in cytotrophoblast cells. Results obtained using immunofluorescence and immunoelectron microscopic detection of endogenous IgG are consistent with the hypothesis that IgG is internalised into clathrin-coated pits and vesicles. Endogenous IgG was not demonstrable in cytotrophoblast cells. The significance of our inability to demonstrate IgG or specific receptor molecules for IgG in cytotrophoblast cells, and possible roles of Fc gamma receptor-bearing cells of the placenta are discussed.